The use of silica as a reinforcing agent in rubber formulations has grown significantly in importance in recent years. In fact, today silica is widely used in rubber formulations as a replacement, or more typically a partial replacement, for carbon black in rubber articles, such as tires. This is because silica reinforcement offers numerous benefits over conventional compounding with carbon black. For instance, U.S. Pat. No. 5,227,425 indicates that tires made with tread formulations that contain silica exhibit a number of important performance advantages over tires manufactured using carbon black as the filler. More specifically, the utilization of silica in tire tread formulations is believed to (a) lower rolling resistance, (b) provide better traction on wet surfaces and snow, and (c) lower noise generation, when compared with conventional tires filled with carbon black.
Sometimes rubber for tires is supplied by a rubber producer to a tire manufacturer in the form of a masterbatch containing an elastomer, an oil extender, and a filler. The traditional filler has been carbon black in the form of fine particles. These particles have hydrophobic surface characteristics and will therefore disperse easily within the hydrophobic elastomer. In contrast, silica has a very hydrophilic surface and considerable difficulty has been encountered in dispersing silica in the hydrophobic rubbery elastomer.
A number of techniques have been developed to incorporate such reinforcing agents and fillers into the polymer compositions, including both wet and dry blending processes. The incorporation of silica and carbon black as reinforcing agents and/or fillers into rubbery elastomers is far more complex than one might initially believe. One problem in wet blending of silica with latices of such polymers arises from the fact that the hydrophilic silica has a tendency to associate with the aqueous phase and not blend uniformly with the hydrophobic polymer.
To overcome the problems associated with the hydrophilic nature of the silica, U.S. Pat. No. 3,317,458 proposed a process whereby an aqueous solution of silicic acid was treated so as to precipitate silica directly onto the surface of an aqueous carbon black in paste form. One significant limitation of this technique is that the surface of the carbon black is altered thus obviating the utility of the many surface structure characteristics of specialty carbon blacks available to the skilled compounder in designing filled, reinforced polymers for specific end uses.
Attempts have been made to use cationic emulsifiers in an effort to distribute such fillers and/or reinforcing agents in polymeric lattices; notably among them are quaternary alkylated ammonium halides (see German Patent No. 1,188,797). It has been found, however, that such quaternary ammonium compounds have a tendency to react with the carbon black, dispersing it into the aqueous phase; that limiting the ability to blend carbon black with the polymeric components in the latex. In U.S. Pat. No. 3,686,113, wet silica is treated with oleophilic quaternary ammonium compounds to blend with an aqueous dispersion of an elastomer containing an anionic dispersing agent. In addition to the problem referenced above with carbon black dispersion, unfortunately, such techniques require specific coagulating agents.
Perhaps the most commonly employed practice used commercially is the technique of dry blending either silica, carbon black or both of them into rubber and thermoplastic polymers in a high-shear milling operation. That practice has many limitations. Notable among them include the tendency of the filler particles to agglomerate to each other, resulting in non-uniform dispersion of the filler throughout the polymer constituting the continuous phase. Another problem commonly experienced in such high-shear milling operations is the tendency of the polymers to degrade during milling. This necessitates the use of higher molecular weight polymers, which sometimes require the incorporation of various types of processing aids to facilitate mixing and dispersion of the filler particles into the polymer constituting the continuous phase. The cost associated with the use of such conventional processing aids also increases the manufacturing cost of the polymeric compound or article and can lead to diminished product performance. The use of conventional processing aids has the further disadvantage in that such processing aids may have a negative effect on the cure or end use of the polymer. Such dry blending techniques also result in additional processing costs, in part due to excessive equipment wear caused by the abrasive fillers.
To improve dispersion of the silica during dry mixing, it has been proposed that such compounding operations employ a silica which has been treated with an organosilane coupling agent having dual functionality. Representative of such coupling agents are those well known compounds that include both an organic group, such as an amino alkyl group, a mercaptoalkyl group, or a polysulfidic-bis-organo alkoxy silane group bonded directly to the silicon atom along with a readily hydrolyzable group, such as an alkoxy group as represented by a methoxy group or an ethoxy group, likewise bonded directly to the silicon atom. In those systems, it is generally recognized that the alkoxy group hydrolyzes in the presence of moisture typically found on the surface of the silica to form the corresponding silanol which reacts with or condenses in the presence of the silica surface to bond the silicon atom to the silica surface. The organic groups likewise attached to the silicon atom are thus available for chemical reaction with the polymer matrix during vulcanization. As a result, the polymer matrix may become chemically bonded by means of the coupling agent to the silica surface during cure or vulcanization of the polymer. Problems associated with the use of such silanes during compounding are unpleasant odors, premature curing, and/or scorching.
In an effort to overcome the problems associated with the use of silane coupling agents, it has been proposed in U.S. Pat. No. 5,405,897 to employ phenoxy acidic acid along with a methylene donor in place of the conventional organosilanes. The foregoing patent suggests that the use of such a system provides improved physical properties and reduced viscosity of the melt during compounding.
Various other attempts have been made to overcome the problems associated with wet blending such fillers and/or reinforcing agents with polymer latices. For example, it has been proposed, as described in U.S. Pat. No. 3,055,956 and U.S. Pat. No. 3,767,605 to add carbon black in the form of a slurry directly to an emulsion polymerization process of rubbery polymer, at the latex stage, followed by coagulation and recovery of a rubber-carbon black masterbatch. Such processes work well with carbon black, but fail to incorporate substantial amounts of fine particulate silica. U.S. Pat. No. 4,481,329 proposes a process for dispersing carbon black and like fillers into concentrated rubber latices by the use of a low molecular weight conjugated diene/carboxylic acid polymer in the form of an alkali metal salt dissolved in water as the dispersing aid or dispersing latex.
U.S. Pat. No. 4,482,657 describes mixtures of silica and synthetic polymers prepared by treating a polymer latex with a dispersion of silica and an alkyl trimethyl ammonium halide in water. The presence of a quaternary ammonium halide in this process necessitates the slow addition of the silica dispersion to prevent premature coagulation. Other elaborate techniques as described in U.S. Pat. No. 3,907,734 where a partitioning agent in the form of a blend of precipitated silica and hydrophobic fumed silica are incorporated into a concentrated polymer latex have been suggested. The fumed silica adsorbs the water, and the resulting solid mixture is dried with removal of the hydrophobic fumed silica to form a free flowing blend of polymer particles coated with precipitated silica. That process is limited to relatively small scale batch system and requires recovery and recycle of the hydrophobic fumed silica. That process fails to incorporate into the polymer the more desirable hydrophobic fumed silica.
U.S. Pat. No. 8,357,733 describes a process for making silica filled rubber masterbatch using silica hydrophobated with a trimethoxy silane coupling agent that is soluble in alcohol-water solution containing at least 70 wt % water. Hydrophobated silica is mixed with latex polymer and incorporated into rubber during the coagulation of the latex. This process has the limitation that it requires that the polymer be an emulsion rather than solution polymerization prepared polymers that may have preferred properties.
Such processes with concentrated latex, as those skilled in the art can readily appreciate, involve complex procedures not only blending the silica with the polymer latex, but also in effecting its recovery when excess silica or carbon black must be employed. Another limitation of such processes is that recovery of the filled polymer directly from the latex stage without filtration and like treatment steps used to remove byproducts from the emulsion polymerization can have deleterious effects on the end use properties of the polymer thus recovered. Such problems can be seen in French Patent 2,558,008 and French Patent 2,558,874. In the first, the addition to a rubber latex of precipitated silica effects coagulation of the rubber polymer. In the second, a stable latex of derivatized silica and a carboxylated butadiene rubber is prepared to add to natural or synthetic elastomer latices. The derivatization of the silica is accomplished by treatment with polyamines, polyethylene amines or nonionic polyoxyethylene. Such free agents are wholly incompatible with typical coagulation techniques used in the recovery of the emulsion process polymers.
It is well known that mercaptosilanes offer excellent coupling between rubber and silica, resulting in rubber compounds for tire treads with improved wet and ice skid resistance, rolling resistance and treadwear even at low loadings. For instance, U.S. Pat. No. 3,768,537 demonstrates the excellent compound properties that can be attained by the use of mercaptosilanes in silica loaded rubber compounds. However, as revealed by U.S. Pat. No. 6,433,065, the high reactivity of mercaptosilanes makes it impractical to use such silane coupling agents in applications where conventional Banbury mixing is employed. In cases where mercaptosilane coupling agents are used in silica compounds it is important to maintain a low temperatures (120° C. to 145° C.) to avoid premature crosslinking which proves to be a problem at higher temperatures. However, low mixing temperatures result in a marked reduction in the mechanical efficiency of mixing that is essential for an optimum dispersion of the silica. The longer mixing time at a low temperature results in a significant reduction in mixing productivity which in turn increases expense. Another drawback of using low temperatures for mixing without extended mixing duration is that less completed silanization occurs which results in the release of ethanol in downstream operations giving rise to porosity from the extrudate and reduced extrusion rates.
Using a combination of two silane coupling agents in silica compounds has been suggested in the patent literature. More specifically, U.S. Pat. No. 6,306,949 discloses the use of a combination of an organosilane disulfide and an organosilane tetrasulfide for silica compounds for enhanced processibility and improved compound properties. In such a process, the organosilane disulfide coupling agent is introduced during the non-productive stage of the mixing so that higher mixing temperatures can be used to mix the silica compounds to ensure a better dispersion of silica throughout the rubber compound. The organosilane tetrasulfide is introduced during the productive stage where the mixing temperature is low (100° C. to 120° C.) so that scorch of the compounds from premature crosslinking can be avoided. However, the very low temperature and short duration during the final pass of the mixing will not ensure sufficient silanization of the organosilane tertrasulfide in the silica compounds during compounding. Since the rate of silanization for tertrasulfide-type silane coupling agents is very low at a temperature lower than 120° C., ethanol is accordingly released during downstream operations, such as extrusions and curing.
U.S. Pat. No. 6,433,065 teaches the use of a small amount of a mercaptosilane coupling agent in combination with an allyl alkoxysilane for silica or silica/carbon black compounds in Banbury mixing. It is claimed that very high temperature mixing (170° C. to 185° C.) can be conducted without causing premature crosslinking of the compounds. U.S. Pat. No. 6,608,145 discloses the use of a small quantity of a organosilane tetrasulfide, bis(triethoxylsilylpropyl)tetrasulfide (TESPT) in combination with an allyl alkoxysilane. It is again claimed that very high temperatures (165° C. to 200° C.) could be used to mix silica or silica/carbon black compounds by Banbury mixing without causing premature crosslinking of the compounds. However, having a non-coupling silane (allyl alkoxysilane) in the silica compounds is not expected to enhance the interaction between silica and the polymeric chain, hence the performance of the silica compounds. U.S. Pat. No. 6,433,065 and U.S. Pat. No. 6,608,145 do not teach the use of mercaptosilane, singly or in combination with allyl alkoxysilane, for the preparation of silica masterbatches in a solvent system.
Different approaches are disclosed in the patent literature for the preparation of silica masterbatches. For Example, U.S. Pat. No. 5,985,953 reveals the preparation of emulsion styrene-butadiene rubber (e-SBR) based silica masterbatches. U.S. Pat. No. 6,433,064 discloses a rubber composition based on emulsion styrene-butadiene rubber and a two step process for making such a composition. U.S. Pat. No. 6,407,153, U.S. Pat. No. 6,420,456, and U.S. Pat. No. 6,323,260 describe processes by which silica particles are first treated with a compound containing amino and silane groups, followed by treatment with a silane compound containing a hydrophobic group. Similarly, U.S. Pat. No. 6,537,612 discloses a process through which the silica particles are treated with different chemical species to render the silica surface hydrophobic. The treated silica is then mixed with solution styrene-butadiene rubber or polybutadiene rubber cement to make elastomer masterbatches. However, an aqueous silica slurry is the starting material for the preparation of the silica masterbatch in all those approaches. Either the silica slurry is prepared by mixing water with silica or an aqueous silica slurry from the precipitated silica production process which is used directly in making silica masterbatches. Silica slurries are used in those approaches on the basis of the conventional thinking that because of the hydrophilic nature of silica, water would be the ideal medium for the treatment of the silica in the preparation of silica masterbatches.
It is well known to those skilled in the art that it is difficult for the silane coupling agents to react directly with silica in an aqueous medium. Hence, transfer agents are disclosed in U.S. Pat. No. 6,465,670 and French Patent 2,804,119 to increase the chance for the silane coupling agent to react with the silica surface. U.S. Pat. Nos. 6,407,153 and 6,420,456 disclose the use of amino silane with alkyl terminations before introducing silane coupling agents. In addition to being a more complex process, the introduction of other chemical species prior to silane coupling agents render some of the reactive sites on the silica surface unavailable for the silanization process.
U.S. Pat. No. 6,025,415 discloses a process through which silica powder could be rendered water-repellent and the dried water-repellent silica could be incorporated into solution elastomer cements in an organic solvent.
There continues to be a long felt need for silica filled rubber formulations that process better (have better extrusion quality) and which exhibit a higher level of dynamic stiffness. However, it is important for these objectives to be attained without compromising other desirable attributes of the silica filler rubber formulation, such as maintaining a low level of hysteresis. The use of silica reinforced tire tread compounds containing organofunctional silanes as coupling agents results in substantial performance benefits, including lower hysteresis and improved wet and ice traction. Unfortunately, these improvements in performance are usually accompanied by difficult tread compound processing due to high Mooney viscosity and reduced tire handling performance due to low dynamic stiffness at low strains of the cured rubber tread. Typical polysulfide silanes used in silica filled tire treads serve to hydrophobate the silica surface, reducing the silica “filler-filler” network resulting in a reduction of dynamic stiffness at low strain levels of the compound. Blocked mercaptosilanes (e.g., 3-octanoylthio-1-propyltriethoxysilane) further amplify this effect. Unlike silica filled tire tread compounds containing polysulfide silanes or blocked mercaptosilanes, carbon black filled tread compounds have high levels of dynamic stiffness at low strain due to the inherent “filler-filler” network formed by the carbon black. This high level of dynamic stiffness at low strain is advantageous for improved tire handling performance. However, this carbon black network also results in a substantial increase in hysteresis as compared to the silica/silane containing tread compounds. Furthermore, the high Mooney viscosity of the silica filled tire tread compounds often require the inclusion of a process additive that reduces the compound viscosity but also further reduces the low strain dynamic stiffness of the cured silica tread compound. Since low strain stiffness of the cured tread compound is a very important parameter for tire handling performance and since process additives reduce low strain dynamic stiffness, a way to increase in the low strain stiffness of a silica filled tread compound without a substantial detrimental increase in hysteresis is needed.